Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (“PDAs”), cellular telephone modems, and other devices. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (“PSTN”) from any place inside the coverage area of the cellular wireless network.
In a Code Division Multiple Access (“CDMA”) wireless network, a mobile station, such as a cellular telephone, ordinarily retains in its memory a set of pseudo-random number offsets (“PN offsets”) that identify base station sectors in the wireless communications network. Typically, the mobile station retains the PN offsets of the three nearest sectors and approximately twenty other PN offsets for the neighboring sectors to these three nearest sectors. The mobile station uses the PN offsets of the three nearest sectors to effect a handoff from one sector or cell of the network to another in its immediate vicinity. Once the network hands off the mobile station to one particular sector or cell, the mobile station obtains another three PN offsets for its new vicinity from the retained twenty numbers for the neighbors.
The mobile station determines which base stations include the three nearest sectors by comparing the strength of pilot signals from the base stations. In some geographical areas, however, a pilot signal from a sector of the nearest base station may be attenuated due to interference from buildings or other RF obstructions. In this case, the pilot signals from farther base stations may have similar signal strengths to the pilot signals of the nearest sectors. The result of this “pilot pollution” is that the mobile station is unable to distinguish the PN offsets of the nearest sectors from the farther sectors and so may attempt to lock onto a farther base station. This erroneous locking may result in the mobile station being dropped from the network as it does not have a sufficiently strong signal to transmit to the farther base station. Also, the mobile station may be unable to resolve ambiguities as to the nearest sectors because it receives the PN offsets that neighbor the farther base station and so perform a series of ineffective handoffs amongst the ambiguous PN offsets, which also results in a dropped call.
Previous solutions to the pilot pollution problem have included antenna down-tilts or base band attenuation at the base stations responsible for the pilot pollution. By directing the radiation pattern of the base station transmitter antenna downwards or curtailing the transmitter power, the pilot signals from the base station preferentially radiates within the base station's sectors and not out to distant geographical areas. In this manner, the strength of the pilot signals is weaker in the area that previously suffered the pilot pollution and so the pilot signals from the nearest sectors in the area are expected to dominate.
But the problem may persist due to diurnal changes in base station transmitter strength, known to those of ordinary skill in the art as “cell breathing.” A property of CDMA systems is that power is shared among the mobile stations. As a cell loads up with mobile stations at particular times of the day, such as rush hour, the radio-frequency (“RF”) interference rises and the cell size effectively shrinks. Thus a nearest sector pilot signal may lose strength as more mobile stations communicate with the nearest sector, and the pilot signal strength may become sufficiently low as to have similar strength to the pilot signal from a farther base station. Therefore, solving the problem with antenna tilt-down or attenuation may work for less loaded times in the day, but not for more loaded times.
It is therefore desirable to provide a method and apparatus for overcoming pilot pollution in an afflicted geographical area covered by the wireless communications system so that mobile stations in the geographical area preferentially lock onto a sector of the nearest base station.